The advantages of wind power are many. It is widely available and does not require flowing water or sources of fuel. Harnessing the wind dates back to the first sailboat. Wind-powered machines have ground grain and pumped water for hundreds of years. With the development of electricity, wind power found new applications in lighting buildings remote from centrally-generated power. Throughout the 20th century, small wind plants evolved for farms and residences, while larger utility-scale wind generators were connected to electricity grids for power in remote locations.
Today, wind powered generators range in size from small plants for battery charging at isolated residences, to expansive offshore wind farms that provide electricity to national electrical networks. Multi-megawatt turbine technologies in use today include advanced aerodynamic, structural, and acoustic engineering design features such as steel tube towers, variable-speed generators, composite blade materials and partial-span pitch control. In 1987, the MOD-5B was the largest single wind turbine operating in the world with a rotor diameter of nearly 100 meters and a rated power of 3.2 megawatts.
However, not all the energy of blowing wind can be harvested, since conservation of mass requires that as much mass of air exits the turbine as enters it. Betz' law gives the maximal achievable extraction of wind power by a wind turbine as 59 percent of the total kinetic energy of the air flowing through the turbine. Further inefficiencies, such as rotor blade friction and drag, gearbox losses, generator and converter losses, reduce the power delivered by a wind turbine. Commercial utility-connected turbines deliver about 75 percent of the Betz limit of power extractable from the wind, at rated operating speed.
Conventional wind turbines face a number of obstacles, including intermittency, space requirements, complaints from homeowners; as such, alternative technologies are being given more serious consideration. One alternative is the airborne generator, the basic premise of which is to tether a device to the ground and let it fly around in the strong winds like a kite, either generating power and sending it down a tether to the ground or using the tether itself to produce electricity at its base.
One example, the Ampyx's PowerPlane, is a glider that generates electricity by pulling on its tether, which is connected to a ground-mounted generator. The PowerPlane glides around between 1,000 and 2,000 feet; the next iteration of this design should generate 250 to 500 kilowatts continuously. There are also inflatable designs, as well as a soft-wing kite design from North Carolina-based Windlift that uses a 40-square-meter wing flying at a maximum altitude of 500 feet, with the controls and generator on the ground.
But scaling up airborne prototypes will not be easy without strong government support. In order to be viable, airborne devices would need to stay aloft for long periods of time with little maintenance required. Another challenge is regulation, since airborne systems are so large and consume such a large volume of airspace.